A strain wave gearing is configured from a rigid internally toothed gear, a flexible externally toothed gear capable of flexing in a radial direction, and a wave generator. Unlike a gear drive composed of a combination of normal rigid gears, a strain wave gearing has no backlash and can yield a large reduction ratio in one stage. The flat strain wave gearing comprising a cylindrical externally toothed gear disclosed in Patent Document 1 is a known example of a typical strain wave gearing.
A flat strain wave gearing comprises two rigid internally toothed gears disposed coaxially and in parallel, a cylindrical flexible externally toothed gear capable of meshing with these internally toothed gears, and a wave generator that causes the externally toothed gear to flex into an ellipsoidal shape and mesh with both internally toothed gears. One first internally toothed gear and the externally toothed gear have the same number of teeth, and the other second internally toothed gear has 2n (n being a positive integer) more teeth, usually two more.
When the wave generator is driven by a motor while the second internally toothed gear having more teeth is secured so as to not rotate, relative rotation corresponding to the difference in the number of teeth is produced in the externally toothed gear. The other first internally toothed gear has the same number of teeth as the externally toothed gear and rotates integrally with the externally toothed gear. Consequently, the rotation of the externally toothed gear is outputted from the first internally toothed gear to a load side.